Logo

I can't see where 'logo' variables gets populated.
And you overwrite the image variable 'start' with millisecs in the function.
I think your loadimage was supposed to be in the 'logo' variable which is to be used in the function calls rather than start...

; sets the graphics mode
Graphics 800,600,0,2
; lets windows know the name of the program
AppTitle "Bounce Back v1.0"
;does everything in the background ready To show when "flip" is used
SetBuffer BackBuffer()
;The globals we'll need to use
Global logo 
;loads the images we'll need to use
logo = LoadImage("bmax160.png")
;Stuff to show the Logo fade in and out
Delay 500
Blend_Black(logo,0,0,0,1000)
time = MilliSecs()+1000
While MilliSecs()<time
	DrawBlock logo,0,0
	Flip
Wend
Blend_Black(logo,0,0,1,1000)
Cls
;while esc. key isn't pressed
While Not KeyHit(1)
;program stuff, nothing goes wrong here
Wend 
End
Function MainMenu()
	Cls
	DrawImage Start,300,125 ;;;;;;;;;;;;;;;;;;Problem is here, image not found
result=0
    While result=0
        mx=MouseX():my=MouseY()
		If RectsOverlap(300,125,200,100,mx,my,0,0) 
		    DrawImage StartOver,300,125 
		Else 
		    DrawImage Start,300,125 ;;;;;;;;;;;;;;;Problem here too, with Start image
		EndIf
	Flip
Wend
End Function
Function Blend_Black(image,x,y,mode,time)
;------------------------------------------------------------------
; Blends an Image with a Black Background (In & Out)
; -----------------------------------------------------------------
; Image = The Image to Blend
; x     = X Coordinate in the Screen
; y     = Y Coordinate in the Screen
;         50000 for both to center image
; Mode  = 0 Blend In
;       = 1 Blend Out
; Time  = Time in Milliseconds
;------------------
	source = ImageBuffer(image)
	dest   = BackBuffer()
	maxx   = ImageWidth(image)-1
	maxy   = ImageHeight(image)-1
	start  = MilliSecs()
	LockBuffer source

	If X = 50000 And y = 50000
		x = (GraphicsWidth()/2)-(ImageWidth(image)/2)
		y = (GraphicsHeight()/2)-(ImageHeight(image)/2)
	EndIf

	While MilliSecs()-start<time
		Cls
		LockBuffer dest
		count=(count+1) Mod 4
		If count=0 Then minx=0: miny=0
		If count=1 Then minx=1: miny=1
		If count=2 Then minx=1: miny=0
		If count=3 Then minx=0: miny=1
		If mode=0 Then value=MilliSecs()-start
		If mode=1 Then value=time-MilliSecs()+start
		For ii=miny To maxy Step 2
			For i=minx To maxx Step 2
				rgb=ReadPixelFast(i,ii,source)
				r=(rgb And $FF0000)/$10000
				g=(rgb And $FF00)/$100
				b=rgb And $FF
				rgb=r*value/time*65536+g*value/time*256+b*value/time
				WritePixelFast x+i,y+ii,rgb,dest
			Next
		Next
		UnlockBuffer dest
		Flip
	Wend
	UnlockBuffer source

	If mode=0 Then DrawBlock image,x,y
	If mode=1 Then Color 0,0,0: Rect x,y,maxx+1,maxy+1
	Flip
End Function